Miniature potentiometer



July 30, 1963 Filed March 18. 1960 H. HABEREDER 3,099,810

MINIATURE POTENTIOMETER 5 Sheets-Sheet 1 l 5o 4I /l/l\ r/ 4 l 44 lo/f26/56 27 50) 52 ha 42/ 4J 5* INVENTR.

HANS HABEREDER BYQDMMML ATTORNEY July 30, 1963 H. HABEREDER MINIATUREPOTENTIOMETER 5 Sheets-Sheet 2 Filed March 18. 1960 FIG. 4

INVENTOR. HANS HABEREDER BYQX MM. M

ATTORNEY July 30, 1963 H. HABEREDER 3,099,810

MINIATURE POTENTIOMETER Filed March 18. 1960 5 Sheets-Sheet 5 FIG. 7

{ e2 I "e3 d. l, 5| HG. e ss/I e\ ill eol|` Il "l 1 JNVE'NTOR HANSuABl-:REDER BYOZM-ML ATTORNEY H.HABEREDER MINIATURE POTENTIOMETER July3o, 1963 Filed March 18. 1960 5 Sheets-Sheet 4 F|G. 8 HANS HABEREDER BYfm. MVM/ ATTORNEY HVVENTOR.

July 30, 1963 H. HABEREDER 3,099,810

MINIATURE POTENTIOMETER Filed March 18. 1960 5 Sheets-Sheet 5 FIG. IO

INVEN ATTORNEY 3,099,810 MlNIATURE PTENTIMETER Hans Habereder, CostaMesa, Calif., assigner to Beckman Instruments, Inc., a corporation ofCaliiornia Filed Mar. 1S, 1960, Ser. No. 15,996 l2 Claims. (Cl.3258-162) This invention relates to potentiometers and more particularlyto potentiometers adapted for miniaturiz-ation.

Miniaturized potentiometers presently known in the art are not adaptedfor low cost of manufacture. They utilize a large number of elementsland require a plurality of m-anuiacturing operations. One reason that acon- Sider-able number of elements are required is that Ifew if any ofthe prior art potentiometers employ elements having dual functions. Theelements used in these potentiometer-s usually provide only a singlefunction; therefore, the number of elements employed is substantiallyequal .to the number of required functions. Prior art potentiometersalso require 'a large amount of labor tor their assembly because of suchmanufacturing operations as cementing the resistance element to -aretaining member and carefully measuring on the resistance element toproperly locate taps thereon, latter which the taps are applied usingspecial tapping equipment.

Another disadvantage of Ithe prior art potentiometers is that theyoftentimes exhibit undesinable electrical characteristics. Somepotentiometers have two or more fric- .tion electrical connections ofpoor design connecting the potentiometer movable contact to anelectrical connection outside .the potentiometer case. These multiplefriction contacts result in a relatively high yand variable resistanceconnection to the movable contact, particularly under conditions ofvibration and shock. Another electrical problem associated with theprior :art devices is that they have `a low leakage resistance betweenthe potentiometer circuitry and the potentiometer housing. Because ofthe problems associated with mounting elements which are movable withrespect to the housing, many prior art potentiometers fail to providecontinuous ydielectric material between the potentiometer circuitry .andhousing.

IContemporary miniaturized potentiometers also lack a suitable means forpreventing movement of the potentiometer movable contact bevond apredetermined point. A stop mechanism is particularly desirable when theresistance element is retained in a circular path. Unless a stop isprovided, the potentiometer contact is permitted to pass over the endsof the resistance resulting in an abrupt change .in resistance 'at thepotentiometer output terminals. Such a sudden change in resistancecannot be tolerated in some applications.

Accordingly, it is an object of this invention to provide apotentiometer of very small size which is inexpensive to manuliaoture.

It is another object of this invention to provide potentiometer elementswhich serve dual functions, thereby minimizing .the .total number ofelements required for constructing miniature potentiometers.

It is still `ano-ther object of invention to reduce the number :ofman-uiacturing operations required to construct miniature potentiometersluy obvia-ting the necessity `for cementing the resistance element to aretaining memher and lby providing a potentiometer in which the accuratetapping of the resistance element is inherently aflorded by thepotentiometer structure.

Another object of this invention is to provide a miniaturizedpotentiometer having improved electrical characteristics including aminimum :and constant resistance path between an electr-ical connectionoutside the potentiometer housing and the potentiometer movable contactunder con- Nnite rates ate- 3,099,810 Patented July 30, 1963 ICC 2lditions of vibration and shock, and very high leak-age resistancebetween the potentiometer cincuitry and the potentiometer housing.

A further object of this invention is to provide an improved stop torthe contact farm of la potentiometer.

Other rand further objects, features and :advantages of the inventionwill become apparent as rthe description proceeds.

Briey, in accordance with a prefenred iorm of the present invention,there is provided a potentiometer which comprises a base mem-ber havingmutually orthogonal cylindrical cavities therein. One of these cavitiesis larger than the other :and has integral therewith a means formounting a resistance element such that electrical contact therewith isfacilitated. A contact mounting gear is also mounted within the ylargercavity and has affixed to it lan electrical contact which is adapted toserve 'as the potentiometric movable contact. A spring of simple designis mounted between the resistance element support and the movablecontact and serves the dual functions of provid ing 1an electrical slipring connection to the movable contact and preventing `the movablecontact from vibrating when the potentiometer is subject to .gneatvibration or shock. A cover is mounted on the base member over thelarger cavity and provides :a notatable support -for the contactmounting gear. A worm gear is mounted in the smaller 'of ltheaiorementioned cavities and engages the contact mounting gear so that larotation of said worm gear will cause a rotation of the movable contact.

This structure permits construction of a potentiometer having improvedelectrical characteristics. The spring member may be directly andrigidly connected to an electrical contact outside fthe potentiometerhousing. Thus, the slip ring connection 'between the movable contact andspring member is the only friction contact in the series path betweenthe external contact and the movable contact. High resist-ance leakagepaths between the electrical circuitry and the potentiometer housing areprovided by widely .sep arating conductive elements of the housing tromthe electrica-l circuitry. As one example, of which others are`described hereinafter, lthe contact mounting lgear is preferablyconstructed from a continuous piece of nonconduotive material. A bossmay be formed on this gear for mating with an opening in the housingcover. The housing cover, which may be an electrically conductivemember, is therefore widely separated from the electrical circuitelements mounted below the spur gear. A significant corollary :of thisrtype of construction is that the housing cover may be constructed byyan inexpensive stamping operati-on instead of la more expensivemachining operation.

Since the present invention ordinarily employs a resistlance 'elementmounted in :a circular path, a continuous rotation of the movablecontact will result in an abrupt change from minimum to resistance. This`type of operation may be undesirable and thereiiore .this inventionprovides a unique lstop for preventing rotation lof the 'movable contact.beyond :a predetermined point, without any undesirable damage orstripping .of the gear teeth.

A more thorough understanding of the invention may he obtained by studyof the following detailed 'description taken in connection with theaccompanying drawings in which:

FIG. 1 is a plan view of a potentiometer embodying the invention inwhich two cutaway portions disclose parts within the potentiometerhousing;

FIG. 2. is a crosssectional view taken along the line 2-2 of FIG. l;

FIG. 3 is a plan view of the resistance coil retainer;

FIG. 4 is a cross-sectional view of the resistance coil retainer takenalong the line 4 4 of FIG. 3;

FIG. 5 is a view in elevation of the contact member;

FIG. 6 is a cross-sectional View of another embodiment of the invention;

FIG. 7 is a plan View of a combined housing and coil retainer;

FIG. 8 is a perspective view of a combined housing and coil retainer;

FIG. 9 is a partial cross-sectional view of another embodiment of thisinvention;

FIG. 10' is a view of a toothless gear adapted for use with thepotentiometer shown in FIG. 9;

FIG. 1l is an exploded view of a stop mechanism constructed inaccordance with this invention; and

FIG. l2. is a plan View of the stop mechanism.

Referring now to FIGS. l and 2, there is shown a preferred embodiment ofthis invention which comprises a potentiometer 10 having a substantiallyrectangular housing 11. The housing may comprise a single rectangularblock of material having a centrally located circular bore formedtherein leaving an internal, cylindrically-shaped cavity 12. Differentmaterials may be used to form housing 11; both die cast aluminum andmolded glass 'lled diallyl phthalate resin have been found to beparticularly adapted for this purpose. Housing 11 is pro-vided indiagonally opposite corners with holes 37 and 38 having axes extendingparallel with the axis of the cavity 12 for mounting the potentiometersingly or in a stack with others.

It will be understood that the potentiometer shown in the `figures isconsiderably enlarged for illustrative purposes. Representative outsidedimensions of potentiometers constructed in accordance with thisinvention are IZII X 1/2!! X B/lll.

An additional cylindrical cavity 13 is located in housing 11 so that theaxes of the cavities 12` and 13 are mutually orthogonal. In order toillustrate this structure more clearly, the internally located cavity 13is shown by dotted lines in FIG. l. A worm gear or lead screw 14 isretained in the cavity to allow teeth of the gear 14 to extend withincavity 12 for engaging the teeth of a contact mounting gear 15. Gear `15preferably comprises a spur gear as shown, although other types of gearsknown in the art may be utilized. Rotation of worm gear 14 causes spurgear 15 to rotate an amount determined by the gear ratio. As shown bydotted lines in FIG. l, worm gear 14 may be conveniently retained inhousing 11 by a pin 21 engaging an annular groove Ztl formed near oneend of the worm gear. Pin 21 is retained in a cavity 22 which is formedin the housing r11 perpendicular to the cavity 13 and located so thatthe edges of the annular groove tly lie within it. Pin 21 is preferablyformed of a resilient material such as polychlorotriiluoroethylene resin`and is force tit into the cavity Z2 to friction load worm gear 142' andprevent rotation of the worm gear when the potentiometer is subiected togreat vibration or shock. A screw drive-r slot 23 may be formed at oneend of worm gear `141 for rotating it from outside the potentiometer.

Spur gear 15 is preferably formed of a resilient material such aspolychlorotriuoroethylene. This material may be readily molded, iselectrically nonconductive and is easily temporarily deformed. A fairlydeep annular groove Z4 is provided for containing a stop mechanism ifneeded. This mechanism will be described in detail hereinafter. Gear 15also has a coaxial circular boss 16 formed on the same surface as groove24. This boss permits gear 15 to be conveniently and inexpensivelymounted by simply drilling a mating orifice 17 in a discshaped member1S. Member 18 also serves as a cover for the potentiometer housing; thismember may conlVeniently take the form yof a thin disc having aperiphery adapted to mate with a groove 19 formed in the sidewalls ofthe cavity 12. Cover 18 may be inexpensively produced by a stampingoperation while spur gear [15 may be completed molded, thus obviatingthe necessity of machining either member. This construction in additionprovides a very high resistance leakage path from the cover to theelectrical circuitry within the potentiometer housing; this isfacilitated by constructing spur gear 15 of a continuous piece ofnonconductive material and directly inserting it between the cover 13and the circuit elements mounted beneath the spur gear.

The potentiometer resistance coil 25 shown in FIG. l is constructed ofresistance wire wound on a core of circular cross section and mounted soas to follow a circular path. An alternative resistance element may beconstructed in accordance with the teachings of an application SerialNo. 700,746, now Patent No. 2,950,996, of Thomas M. Place, Sr., et al.,entitled Electrical Resistance Material and Method of Making Same, iiledDecember 5, 1957, and assigned to Beckman Instruments, Inc., assignee ofthe present invention. This application discloses a ceramic type ofresistance material which may be applied to la nonconductive refractorybase. This type `of resistance element is particularly adapted for usein high temperatures. A resistance coil retainer 27 is mounted in thebottom of cylindrical cavity 12 and may be affixed therein `by a closefit. This type `of construction will be required whenever the housing 11is constructed of an electrically conductive material. When the housing11 is itself electrically insulative, the coil retainer Z7 may be formed`within the casing itself. A modified p0- tentiometer so constructed isdescribed hereinafter.

The coil retainer 27 is shown in detail in FIGS. 3 and 4. This elementis also preferably molded from a resilient material such aspolychlorotrifluoroethylene, and comprises essentially, a disc-shapedmember with the annular groove 26 and ta slot 33. The groove 26- ispreferably molded with an undercut portion 314 so that the resistancecoil may be merely snapped into place and retained securely without theuse of cement. This type of construction avoids the necessity `of anydrying or curing of the cement. As described more fully hereinafter, theslot 33 permits access to the ends of the resistance element from theside of the coil retainer, and thus facilitates the connection ofexternal leads to the resistance element. A groove 35 is provided in theside of the coil retainer `opposite the side in which annular groove 26is located. As noted below, an electrical connection to `another portionof the resistance coil may be provided by placing an electricalconductor in this groove `and contacting the resistance coil through anopening 36.

The movable electrical connection to the resistance coil 25 is provided`by a contact arm 28. As shown in FIG. 5, this arm is preferablyconstructed lfrom a single metal spring strip. Referring again to FIG.2, this arm has a central portion atiixed to the spur gear 15 so as tobe rotatable therewith, and an elongated portion which makes electricalcontact with the resistance coil 2.5. A convenient means of mountingthis contact arm to the spur gear is to form a hexagonally shapedprotrusion -29 (shown in FIG. 5) in the contact arm which is force fitinto a circular opening in the spur gear 15. Because of the resiliencyof the spur gear, the opening therein conforms to the noncircularprotrusion thus providing a rigid, yet inexpensive connection. Foradditional securing of the Contact arm to the gear, spring 3@ producesan upward force to hold the arm into the gear.

Spring 3d is a curved disc of conductive spring-like material. Thespring with its concave side downward lies in a suitable circulardepression 31 formed in the coil retainer 27. The spring is preventedfrom rotating by a tap -32 which fits into the slot 33 in the coilretainer.

The convex side of the spring 30 abuts the contact arm 2S and providesan electrical slip ring connection to the contact arm 28, as well asholding the arm into the spur gear. Because of the large area of contactbetween the spring Sil and the contact arm 28 a very low resistanceconnection is possible. Also of importance is that this connectionmaintains a constant low impedance as the contact arm is rotatedrelative to the spring. Moreover, the connection between the contact armand the spring prevents the contact arm and gear from vibrating when thepotentiometer is subject to great vibration or shock. Thus, althoughspring `3J() is `an inexpensive and easily manufactured item, itnonetheless provides the dual functions lof forming an electrical slipring connection and preventing vibration in a very satisfactory manner.

Electnical connections from the outside of the potentiometer are madevia a cylindrical plug 40 molded from insulative material. Lead wire 41has a bared end 42 inserted in a cylindrical pocket 43. A U-shaped bendin the wire permits it to be passed through a circular orice 4liin theplug 40'. Lead wire `i5 is similarly aflixed in plug 40. The `U-shapeformed in this lead wire in combination lwith the oriiices of the plug40 provides the important functions of retaining the lead wires in theplug very securely without further mechanical fastening and positioningthe resistance coil taps very accurately. As shown in FG. 1, the plug40* is inserted in a mating cylindrical opening 46 in the housing 11.Shoulders 4S yformed on the plug determine .the depth to which the plugextends into the housing. As shown in a cutaway portion of FIG. l, theslot 33 in the coil retainer exposes the ends of the resistance element25 so as to permit access thereto from the side of the retainingelement. With the plug 4t) inserted into the side opening 46, the endsof the electrical leads 411 and 45 will either touch or be in closerelationship to a respective end of the resistance coil 25. A drop ofconductive cement may be applied to each junction to assure goodelectrical contact between the leads and the resistance element. The tappositions on the resistance element are controlled very accurately bythe distance between the molded pocket 43 and the other molded pocketfor lead 45 (not shown). The means afforded by the plug 40 for tappingthe resistance element is a significant advance in the art. Previously,accurately positioning the tap positions required the use of carefulangular measurements and special tapping equipment.

The electrical connection to the spring disc 30 comprises lead wire 47welded to the spring tap 32 prior to assembly of the spring in thepotentiometer. An additional hole in the plug 4t) allows -lead 47 toextend to the outside of the potentiometer. This positive connectionbetween the contact outside of the potentiometer housing and the springinsures a minimum and constant series resistance path from the outsidecontact to the potentiometer movable contact. In this series path thereis only a single friction contact, i .e., between the spring and themovable contact. And as noted above, this connection has been designedto give a minimum and constant resistance.

A tap to the center or any other preferred position of the resistancecoil 25 may be provided by placing a conducting wire or a flat metalstrip Stlfin the groove 35 in the bottom of coil retainer 2.7. When thehousing 11 is constructed of a conductive material, the conductor 5t)will, of course, ybe insulated therefrom by appropriate insulationplaced between the conductor and the housing. As shown in FIG. 2, oneend of this strip passes through the opening 36 in the coil retainer tomake electrical contact with the resistance coil 25 at the appropriateposition, and the other end of the strip is welded to one end of leadwire 5,1. When the center tap is desired, plug 40 will include anadditional hole through which the center tap lead 511 may be passed tothe outside of the potentiometer.

The potentiometer construction is completed by fastening the cover andplug to the rectangular portion of the potentiometer housing. As shownin FIG. l, V-shaped groove 52 cut in a circular path in the top of thehousing 11 permits the cover to be fastened by staking or folding aperipheral edge of the groove 19 over the cover 18. In similar manner,edges 53 and 54 of a rectangular slot in the corner of housing 11 are-folded over the rectangular edges of the plug shoulders 4S to fastenplug 40 to the housing 11.

An examination of the potentiometer of the invention will reveal severalreasons ffor its improved electrical characteristics. As describedabove, the novel construction affords a minimum and constant resistancebetween the contact outside of the potentiometer housing and thepotentiometer movable contact. Also described above is the spur gear andassociated mounting cover for materially increasing the leakageresistance between the electrical circuit elements within thepotentiometer and the potentiometer housing. Referring particularly nowto FIG. 2, it may be observed that coil retainer 27 provides a wideseparation between the resistance element 25 and housing 111. Also, themetal worm gear 14 is situated so .as to be Widely separated from theresistance coil 25 and movable contact 28, and likewise a wideseparation is maintained between the movable contact and thepotentiometer housing. These wide separations materially increase theresistance of the leakage paths to the potentiometer housing andsubstantially improve the electrical characteristics thereof.

An `alternative embodiment of this invention is illustrated in FIGS. 6,7 and 8. In this version of the potentiometer, the coil retainer and aportion of the housing comprise a single molded element. Thisalternative embodiment is also provided with terminals adapted for usewith printed circuitry. However, it will be obvious to -those skilled`in the art that external circuit connections can be made by lead wiresin the manner heretofore shown, or that the printed circuit terminalsshown hereinafter may be used in the potentiometer described above.

Referring now to FIG. 6, the potentiometer housing includes base element55 and cover element 56 each preferably molded of a plastic such asglass filled diallyl phthalate resin. Base element 55 is shown in planview in FIG. 7 and perspective view in YFIG. 8. Referring to these threefigures, base `element 55 is molded so as to provide a half cylindricalcavity 57 which mates with a like hal-f cylindrical cavity 54 molded inthe cover member 56 :for retaining the worm gear. rIlhis constructionobviates machining =or :drilling `on either the base element or coverelement thereby reducing the cost. In this embodiment, as in the onedescribed hereinbefore, it is desirable to friction load the worm gear.This may be very conveniently accomplished by inserting a resilientrectangular element 65 (see FIG. 8) constructed preferably frompolychlorotriuoroethylene into a rectangular 4groove 6,6 molded into thebase element 55. When so inserted, the resilient block 65 extendssomewhat into the Space in 'which the worm gear will be located, asdenoted by fthe dotted lines in FIG. 8. With the worm gear and resilientblock inserted and the cover element and :the base element joined, theresiliency of the block 65 will cause it to deform to provide thedesired function loading. 'I'he worm .gear will then be substantiallyunaffected by very high shock and vibration.

A groove 58 following a circular paith` is molded in the base element55- for providing the resistance coil retainer. Terminal pins 59, y60and `61 are retained in respective orices 62, 63 and 64 in the basemember 55. Rectangular grooves 70 and 71 are molded in the element 55-for lretaining respective Ithin metal strips 72 and 73 for connectingthe terminal pins to appropriate taps on the resistance elements. Thelgrooves 70 and 71 are formed at the appropriate angle from a contactpin -to the coilretaining groove 5S so that the resistance coil isrtapped at the required points. This construction affords an accuratepositioning of the lresistance taps since the metal strips are merelylaid in the rectangular grooves after which lthe resistance element isfitted in the coil retaining groove 58 and cemented tothe metal stripsat the tap positions. The tap placement is molded automatically at thetime the base element is formed and no additional drilling or measuringis required. Terminal pins 59 and i60* are respectively connected to theftwo flat metal strips 72 and 73, such connections being shown in iFIG.6. Terminal pin 61 is connected to the spring element which may beidentical to spring 30 previously shown and described. A small wireyformed of, for example, nickel maybe used to connect the terminal pin61 to the spring.

The cover element 56 is a molded element =of simple construction. Thecontact pins, resistance element, spring, contact arm, worm gear andspur gear are each placed in the position shown in PIG. 6 after whichthe cover element 56 is placed in the position shown and axed to thebase element by any suitable means. These elements, shown in FIG. 6 butnot described in detail, may be identical in construction and functionas the elements previously shown and described in FIGS. l and 2.

A still further embodiment of this invention is shown in FIGS. 9 and l0.IFIG. 9 shows just the upper portion of a cutway view of apotentiometer; the remainder of the potentiometer may be identical tothat of fthe embodiments hereinbefore shown and described. As shown inthis figure, the disc-shaped element 73 has only a slightly smallerdiameter lthan that o-f the cylindrical cavity in (the (upper or coverelement 74 so that the element will rotate about a substantially fixedaxis relative to the housing. The .element 73 is preferably formed of alow friction material so that it will smoothly ride within the housing74. As shown in this structural form, the housing does not require anyopening for mounting the element 73.

An additional feature of this embodiment is that driven element 73 mayIbe a simple disc ywithout any teeth formed thereon if it is constructedfrom a suitable resilient material. The preferred material for thisapplication is polytetrauoroethylene resin. This material has a muchlower coeicient of friction than other resilient materials such aspolychlorotrifluoroethylene, and is also more easily temporarilydeformed. Thus, -when the worm :gear 14 and the element 73 are in thepositions shown in FIG. 9, disc-shaped :element 73 is temporarilydeformed to allow the worm [gear teeth to engage the disc. As the wormgear 14 is rotated, element 73 will likewise rotate in fthe same manneras i-f teeth had been preformed on the element since new teeth will betemporarily 'formed on element 73 adjacent the worm gear.

The potentiometers shown and described heretofore are so constructedthat :the worm gear may be continuously rotated in a given direction.Since the resistance element is laid in a circular path, .this type ofconstruction causes the resistance connected .to the output terminals toabruptly change from a minimum to a maximum value, as the contact armpasses over the gap between the ends of resistance elements. In somecircumtances, such an abrupt resistance change may not be desirable ortolerable. Therefore, a potentiometer having the features enumeratedabove with the additional feature of a unique stop for the movableContact arm is described hereinafter.

An exploded View of the stop mechanism is shown in FIG. 1l. Worm gear 14may be identical to the worm gears used in the prior potentiometerembodiments and, as before, this worm gear mates with a spur gear. '1`hespur gear 75 illustrated in FIG. 7 is similar to spur gear 15 previouslyshown with the exception that a plurality of teeth are removed, leavinga blank base 76 and a rectangular support 90. Spur -gear 7S funtherincludes the annular groove 24 formed in the top side of the 'gear intowhich is inserted a stop disc 78. 'I'he stop disc is substantiallywasher-shaped with a projecting member 79 bent at right agnles to thedisc and having .tabs 80 and 81 bent at an angle to member 79.Projection 79 is placed upon the support 90 with the ta'bs 80 and 81respectively adjacent the end teeth 82 and `83 of the spur gear 75.These tabs, when not deformed, lie in approximately the same position asnormal teeth adjacent the respective end teeth 82 and 83, as shown bythe dotted teeth in FIG. 12. An additional protruding tab 84 is lbent atright angles to the surface of the disc.

The stop disc 78 may be formed from a flat piece of thin sheet material;preferably, the material selected is springlike in nature so that thetabs Si) and 81 may be bent slightly without permanently changing theirangular relationship to the projection 79. In order to permit the tabsto be bent while minmizing the breakage thereof, the member 79 isundercut at 91 and 92. thus lengthening the effective bending arm foreach of the tabs Si) and Sil. An a'didtional element of the stopmechanism shown in FIG. l2 is a stop boss 87 affixed to thepotentiometer case.

The operation and assembly of the stop mechanism is illustrated in FIG.l2. The stop boss 87 is rigidly attached Ito the potentiometer case andpreferably is integrally formed on the bottom surface of thepotentiometer feover. When the worm .gear 14 is rotated in acounterclockwise direction as denoted by the arrow 35, the spur gearwill rotate in a clockwise direction as denoted by the arrow 86. Whenthe tab 84 contacts the stop 87, tooth 83 of the spur gear will havejust moved beyond the worm gear teeth. There is, therefore, no forceapplied -on the spur gear teeth in the stop position thus preventingstripping of `or damage to the teeth.

In the position shown in FIG. 12, tab 81 serves as an escapementmechanism; each rotation of the worm gear in the direction of arrow 85causes the tab to bend from its normal position (shown in dotted lines81a) and then spring back to its former position. When, however, theworm gear is rotated in a clockwise direction, denoted as arrow 88, aforce will be applied to the tab 81a in an opposite direction. Since thespur gear 75 is not retained by tab 84 abutting stop boss 87 in thisdirection, spur gear 89 will be caused to rotate in a counterclockwisedirection as denoted by arrow 89. Immediately following tab 81 is a spurgear tooth S3 which engages the worm gear 14. The worm gear and spurgear now operate in the normal manner until the stop disc tab 84 abutsstop boss 87 in the Opposite direction. Tab 80 of the stop disc thenprovides the escapement mechanism in the same manner as was heretoforedescribed in reference to tab 81. It will be understood that the stopmechanism may be provided for any position of the contact arm bysuitably positioning stop boss 87 relative to tab 84. Ordinarily, ofcourse, tab S4 and stop boss 87 will be positioned relative to eachother so .that the movable contact will not cross the gap separating theends of the resistance element.

Although exemplary embodiments of the invention have been disclosed anddiscussed, it will be understood that other applications of theinvention are possible and that the embodiments disclosed may besubjected to various changes, modilications and substitutions withoutnecessarily departing from Ithe spirit of the invention.

I claim as my invention:

l. A variable resistance comprising a housing, a resistance elementmounted within said housing, a contact member engaging said resistanceelement, means for rotatably mounting said contact member including acontact mounting gear rotatably mounted with respect to said housing andin fixed relationship with said contact member, a worm gear mounted insaid housing in engagement with said gear, said worm gear having anannular groove extending into a cavity formed within said housing, andmeans formed of a resilient material such as polychlorotriiluoroethyleneresin retained in said cavisty and deformed by the face .of said annulargroove for friction loading the worm gear.

2. A variable resistance comprising a housing, a resistance elementmounted within said housing, a `contact member engaging said resistanceelement, a member formed of resilient material rotatably mounted withinsaid housing, and a hexagonally shaped protrusion formed in said contactmember which engages `and deforms a circular opening in said rotatablemember.

3. In a variable resistance, a housing, |a resistance element, and meansfor mounting said resistance element within said housing comprising ablock of resilient material having an annular groove formed in a facethereof, said groove being undercut with said face extending over aportion thereof so that said resistance element may be snapped intoplace and retained securely without the use of cement.

4. ln ia potentiometer having a movable contact aixed to a gear, and aWorm gear in driving relationship with said gear, the improvementcomprising a potentiometer housing having a pair of mated members eachhaving formed therein a half cylinder, the half cylinders in each memberbeing located opposite each other when the members are fixedly mountedtogether so as to form a complete cylinder for retaining said worm gear,and means for friction loading said Worm gear comprising an elementcomposed of a resilient material such as polychlorotrifluoroethyleneresin integral with one of said half cylinders and in engagement withsaid worm gear.

5. A variable resistance comprising a housing having a base and a topmember, a groove formed in the upper face of said base member whichfollows a substantially circular path, an electrical contact mounted insaid base member within the area bounded by said circular groove, agroove in the upper face of said base member which follows asubstantially linear path extending from said electrical contact to apredetermined point of said circular groove, a resistance elementretained in said circular groove, electrically conductive means retainedin said linear groove and in contact with said resistance element atsaid predetermined point, and contact means electrically contacting saidresistance and rotatably mounted to said housing.

6. A variable resistance comprising a housing having top, bottom andside walls; `a coil retainer comprising a substantially disc-shapedmember of insulating material fixedly mounted to said bottom wall, saidcoil retainer having a groove in its upper face which follows asubstantially circular path, a resistance element fixedly mounted insaid groove; contact means electrically contacting said resistance androtatably mounted to said housing about a first axis perpendicular tosaid -top and bottom faces; said coil retainer having a slot thereinexposing the ends of said resistance element, a circular orifice in oneof said side walls facing said slot, a cylindrical plug mounted in saidcircular orifice having a first orifice and a first pocket spaced alonga second axis substantially perpendicular to `said bottom and top walls`and a second orifice and a second pocket spaced along a third axissubstantially perpendicular to said bottom and top walls, said secondand third axes being located adjacent the points of desired contact withthe `ends of said resistance element, a first electrical lead passingthrough said first orifice, the end of said first lead being bared, bentin a U-shape and inserted in said first pocket; and a second electricallead passing through said second orifice, the end yof said second leadlbeing barred, bent in a U-shape and inserted in said second pocket.

7. A variable resistance comprising a housing; a resistance elementmounted within said housing; a contact member engaging said resistanceelement; means for rotatably mounting said contact member including adisc of resilient, easily temporarily deformable material characterizedby a low coeiiicient of friction, said disc being mounted with acylindrical cavity formed within said housing and in fixed relationshipWith said contact member, said cavity having a diameter only slightlylarger than the -diameter of said disc so that the disc may rotate 10about a substantially fixed axis relative to the housing, and a wor-mgear retained in said housing so that its teeth engage and temporarilydeform a peripheral portion of said disc.

8. The variable resistance element defined in claim 7 wherein said discis formed of polytetrafluoroethylene.

9. A variable resistance comprising a housing; a resistance elementmounted within said housing, a contact mounting gear rotatably mountedwithin said housing, a worm gear mounted in said housing in engagementwith said contact mounting gear, .a Contact member aiiixed to saidcontact mounting gear and engaging said resistance element, anelectrically conductive spring comprising a substantially discshapedmember having an arcuate cross section, the convex side of said springabutting said contact member and the concave side of -said springabutting and iixedly retained by said housing, said spring making aslip-ring electrical contact with said contact member and preventingsaid contact member and rotatable member from vibrating.

l0. A variable resistance comprising a housing; a resistance elementmounted within said housing; a contact mounting gear comprising acontinuous disc of resilient nonconductive material having a circularboss located coaxially with the axis thereof, said boss being rotatablymounted in a mating orifice extending entirely through a wall of saidhousing; a worm gear mounted in said housing in engagement with saidcontact mounting gear; a contact member aflixed to said contact mountinggear and engaging said resistance element, yand an electricallyconductive spring fixedly retained by said housing and engaging saidcontact member for making a slip-ring electrical contact to said contactmember and preventing said contact member and rotatable member fromvibrat- 1l. A variable resistance comprising a housing includingsubstantially parallel top and bottom walls and a cylindrical cavityformed therewithin with the axis of Said cavity being substantiallyperpendicular to said top and bottom Walls; a resistance element mountedwithin Said cavity nearest said bottom wall; a movable contact member;means for rotatably supporting said contact member and providing acontinuous insulative cover of high resistance between said top wall andsaid resistance element and movable contact including a gear comprisingta continuous disc of resilient nonconductive material, said movablecontact being in fixed engagement with said gear on one face thereof,said gear being rotatably mounted within said cavity with said one faceopposite said resistance element and the other face of said gearopposite said top wall, means including an electrically conductivespring mounted between the bottom wall of said housing and said gear formaking an electrical connection between the outside of said housing andsaid contact member, and -a Worm gear mounted in said housing andengaging said gear.

L2. The variable resistance defined in claim 11 wherein said gearincludes a circular boss located coaxially with the axis thereof, saidboss being rotatably mounted in a mating orifice extending entirelythrough said top wall.

References Cited in the file of this patent UNITED STATES PATENTS2,477,711 Youhouse Aug. 2, 1949 2,596,503 Newnham May 13, 1952 2,880,293Blanco Mar. 31, 1959 2,952,825 Bourns Sept. 13, 1960 2,968,015 BlancoJan. 10, 1961

1. A VARIABLE RESISTANCE COMPRISING A HOUSING, A RESISTANCE ELEMENTMOUNTED WITHIN SAID HOUSING, A CONTACT MEMBER ENGAGING SAID RESISTANCEELEMENT, MEANS FOR ROTATABLY MOUNTING SAID CONTACT MEMBER INCLUDING ACONTACT MOUNTING GEAR ROTATABLY MOUNTED WITH RESPECT TO SAID HOUSING ANDIN FIXED RELATIONSHIP WITH SIAD CONTACT MEMBER, A WORM GEAR MOUNTED INSAID HOUSING IN ENGAGEMENT WITH SAID GEAR, SAID WORM GEAR HAVING ANANNULAR GROOVE EXTENDING INTO A CAVITY FORMED WITHIN SAID HOUSING, ANDMEANS FORMED OF A RESILIENT MATERIAL SUCH AS POLYCHLOROTRIFLUOROETHYLENERESIN RETAINED IN SAID CAVITY AND DEFORMED BY THE FACE OF SAID ANNULARGROOVE FOR FRICTION LOADING THE WORM GEAR.